For many years, as discussed in Andrews, U.S. Pat. No. 4,837,383 the automobile industry has manufactured automobiles with back windows which include electrical heating elements to remove frost formed on the window surface. The back windows are printed by a silkscreen printing process with a grid of a metallic material which is then fired on the glass window to form the electrical heating element. In most instances, the grid arrangement forming the heating element is comprised of a bus bar extending along each side of the window, and a series of fine lines extending horizontally across the window, with the fine lines being connected to the bus bars. The grid material from which the heating element is formed typically comprises a mixture containing a silver powder and a small amount of soft lead glass dispersed in a printing vehicle, such as oil, suitable for silkscreen printing. The grid material is applied to the glass substrate in a silk-screen printing process.
Also frequently applied to the back window is a dark grey or black enamel border extending about the periphery, or outer edge, of the back window. The border is printed both for aesthetic reasons, and, also, to protect an adhesive utilized to seal the back window to the car body from degradation due to exposure to ultraviolet light.
In some instances, the bus bars are printed over the enamel border, but, after firing, the bus bars are still visible, and appear from the outside of the automobile, for example, as a dark amber color.
In order to prevent the detection of the bus bars from outside of the automobile some prior art patents disclose altering the composition of the enamel utilized to form the border. In particu-lar, some prior art patents disclose the addition of powdered zinc, tin, cadmium, or manganese to the enamel. Some prior art patents also suggest the addition of powdered metals such as zinc, tin, cadmium or manganese to the enamel, paint or die to facilitate the forming of the glass substrate by helping to prevent the sticking of the forming head or die to the paint or enamel.
More particularly, disclosed in U.S. Pat. Nos. 4,684,388 and 4,684,389 are means to form a glass sheet having an oil base paint fired thereupon wherein the paint contains a fine zinc metal powder. U.S. Pat. No. 4,684,388 further discloses the inclusion of a fine stannous oxide powder in an ultraviolet curable paint which, when applied to the glass sheet, is subject to ultraviolet radiation and heated to a temperature to soften the glass sheet to allow bending thereof. The paint on the glass engages with a fiberglass covering on a forming head or die. The zinc and the stannous oxide powders serve to prevent the sticking of the paint to the fiberglass forming die. U.S. Pat. No. 4,684,389 discloses an oil base paint to which fine zinc powder is added to the paint applied to the glass sheet. The painted glass sheet is then heated to a forming temperature and engaged with a fiberglass covering of a die to form a glass sheet of a desired shape. The metal powder functions to prevent the sticking of the paint to the fiberglass of the forming head or die during the forming process.
Boaz, U.S. Pat. No. 4,596,590 discloses a method of forming a glass sheet with a paint that minimizes sticking. The paint includes a metal oxide powder having at least a low valence oxidation state and a high valence oxidation state, the metal oxide powder being in its low valence state when applied. Examples of suitable metal oxide powders include stannous oxide, iron oxide and cuprous oxide.
Stotka, U.S. Pat. No. 4,983,196 also discloses an enamel composition that minimizes sticking. The enamel includes an iron metal powder to help prevent adhesion during the forming operation.
Andrews et al., U.S. Pat. No. 4,975,301 discloses a glass enamel which serves to help hide the bus bars of the heating element. The enamel disclosed by Andrews et al. comprises powdered zinc, tin, cadmium, manganese, iron and mixture and alloys thereof for use in conjunction with a soda-free flux glass.
Andrews, U.S. Pat. No. 4,837,383 also discloses a glass enamel which serves to help hide the bus bars of the heating element. The enamel disclosed by Andrews includes aluminum or lithium oxide.